Exhaustive Study Notes on Neurotransmission and Neurotransmitter Systems

Foundations of Neurotransmission and Neurotransmitters

• Neurotransmitters (NTs) are chemical messengers that relay, amplify, and modulate electrical signals between neurons or between a neuron and an effector cell.
• The human nervous system utilizes more than 100 distinct chemical neurotransmitters, broadly categorized into small-molecule neurotransmitters and neuropeptides (ranging from 3 to 36 amino acids in length).
• Neurotransmitters act locally at:
  • Postsynaptic receptors and ion channels to alter postsynaptic membrane potentials.
  • Presynaptic receptors and ion channels (autoreceptors and heteroreceptors) to regulate further neurotransmitter synthesis and exocytosis.
• Cellular localization of neurotransmitters includes:
  • Presynaptic synaptic vesicles and dense-core vesicles within nerve terminals.
  • The synaptic cleft following activity-dependent exocytosis.

Essential Criteria for Identifying Neurotransmitters

• Identity / Presence: The candidate substance must be present within presynaptic neurons (e.g., demonstrated via immunohistochemical staining such as tyrosine hydroxylase [TH] staining).
• Synthesis: The presynaptic neuron must possess the necessary precursor molecules and biosynthetic enzymes required to synthesize the substance.
• Release: The substance must be released from presynaptic terminals in a stimulus-dependent manner (concomitant with presynaptic action potentials) and in sufficient quantities to exert a physiological effect on the postsynaptic cell. Evidence is collected via synaptic cleft sampling, microdialysis, or electrophysiological recording of Excitatory Postsynaptic Potentials (EPSPs) and Inhibitory Postsynaptic Potentials (IPSPs).
• Inactivation: An active mechanism for termination of signaling must exist, operating through enzymatic degradation, active neuronal/glial reuptake, or both.
• Receptor Interaction: The substance must interact with specific functional receptors on the target postsynaptic or presynaptic cell membrane.
• Pharmacological Profile:
  • Exogenous application of agonists must fully mimic the functional actions of the endogenous neurotransmitter.
  • Specific antagonists must competitively or non-competitively block the physiological actions of both the endogenous neurotransmitter and exogenous agonists in a concentration-dependent (affinity and potency-related) manner.
  • Applied substances must produce identical effects on ion channel gating, membrane input resistance, and membrane potential, and must demonstrate cross-desensitization with the endogenous transmitter.
  • The candidate substance must produce these biological responses at physiological concentrations.

Classification of Neurotransmitters

• Neurotransmitters are classified functionally by their net effect on the postsynaptic membrane potential or structurally by their chemical backbone.

Functional Classification

• Excitatory ($E = +$): Depolarizes the postsynaptic membrane, increasing the likelihood of generating an action potential.
• Inhibitory ($I = -$): Hyperpolarizes the postsynaptic membrane or stabilizes membrane potential, decreasing the likelihood of action potential generation.

Structural Classification

• Small-Molecule Neurotransmitters:
  • Acetylcholine (ACh): Primary excitatory transmitter at neuromuscular junctions and autonomic ganglia.
  • Amino Acids:
  • Excitatory ($+$): Glutamate, Aspartate.
  • Inhibitory ($-$): $\gamma$-Aminobutyric acid (GABA), Glycine.
  • Biogenic Amines (Monoamines):
  • Catecholamines ($+$): Dopamine (DA), Norepinephrine (NE / Noradrenaline), Epinephrine (E / Adrenaline).
  • Indoleamines ($+$): Serotonin ($5\text{-HT}$ / 5-Hydroxytryptamine), Melatonin.
  • Imidazoleamines ($+$): Histamine.
  • Purines ($+$): Adenosine, Adenosine Triphosphate (ATP).
  • Gases ($+\text{ and } -$): Nitric Oxide (NO).
• Neuropeptides ($+\text{ and } -$):
  • Chains of 3 to 36 amino acids.
  • Examples: Endorphin, Enkephalin, Neuropeptide Y, Substance P, Oxytocin, Somatostatin.

Neuropeptides Characteristics

• Synthesized as precursor proteins in the soma, processed through the endoplasmic reticulum and Golgi apparatus, and transported down the axon.
• Stored in axon terminals within large, electron-dense vesicles termed dense-core vesicles.
• Functionally active at significantly lower concentrations ($10^{-9}\,\text{M}$ to $10^{-11}\,\text{M}$) compared to small-molecule transmitters.
• Elicit longer-lasting neuromodulatory effects.
• Some neuropeptides are co-released from non-neuronal tissues (e.g., gut-brain peptides regulating appetite and food cravings).
• Act as circulating neurohormones or as co-transmitters that modify classic small-molecule neurotransmitter actions.

General Mechanisms of Drug Action at Synapses

• Pharmacological agents and toxins modify synaptic transmission by targeting distinct biochemical and physiological steps:
  • Receptor Interaction: Direct activation (agonism) or blockade (antagonism) of postsynaptic or presynaptic receptors.
  • Vesicular Storage: Inhibition of vesicular transporters, leading to depletion of neurotransmitter stores or enhanced non-vesicular efflux.
  • Reuptake Transporters: Blockade of plasma membrane reuptake transporters, elevating synaptic neurotransmitter concentrations and prolonging receptor activation.
  • Neurotransmitter Synthesis: Augmentation (via precursor loading) or inhibition (via enzyme inhibition) of neurotransmitter biosynthetic pathways.
  • Neurotransmitter Release: Facilitation or blockade of vesicular exocytosis (e.g., clostridial neurotoxins cleavage of SNARE complex proteins).
  • Ion Channels: Direct alteration of voltage-gated ion channels, such as voltage-gated $Ca^{2+}$ channels (calcium channel blockers) or voltage-gated $Na^+$ channels (local anesthetics, tetrodotoxin).

The Cholinergic System and Acetylcholine

• Acetylcholine (ACh) was the first chemical substance identified as a neurotransmitter.

Anatomical Distribution and Pathways

• Peripheral Nervous System (PNS):
  • Somatic nervous system motor neurons projecting to skeletal muscle at the neuromuscular junction (NMJ).
  • Pre-ganglionic sympathetic and parasympathetic neurons in autonomic ganglia.
  • Post-ganglionic parasympathetic neurons terminating at smooth muscle, cardiac muscle, and glandular tissues.
  • Parasympathetic neuroeffector junction formed by the vagus nerve onto cardiac sinoatrial (SA) and atrioventricular (AV) nodes.
• Central Nervous System (CNS):
  • Midbrain Cholinergic Complex: Pedunculopontine tegmental (PPT) nucleus and laterodorsal tegmental (LDT) nucleus project diffusely to the thalamus, basal ganglia, and prefrontal cortex.
  • Basal Forebrain Cholinergic System: Nucleus basalis of Meynert and medial septal nuclei project extensively to the neocortex and hippocampus; critical for linking unconscious physiological state to conscious awareness, processing novel stimuli, and directing focused attention.
  • Functions: Maintenance of the sleep-wake cycle, regulation of sensory gating, memory encoding, motivation, and arousal.

Acetylcholine Receptors

• Cholinergic receptors are categorized into ionotropic nicotinic receptors and metabotropic muscarinic receptors.

Nicotinic Acetylcholine Receptors ($nAChR$)

• Type: Ligand-gated, non-selective cation channel ($Na^+$ and $K^+$ permeable).
• Subtypes:
  • $N_N$ (Neuronal-type): Located in autonomic ganglia, the adrenal medulla, and the CNS.
  • $N_M$ (Muscle-type): Located at the skeletal neuromuscular junction (post-junctional membrane).
• Molecular Structure:
  • Pentameric complex assembled from 5 protein subunits ($2\alpha, 1\beta, 1\delta, 1\epsilon$ in adult muscle, or $2\alpha_1\beta_1\epsilon\delta$; neuronal types are typically $2\alpha_4 3\beta_2$ or homomeric $5\alpha_7$).
  • Each subunit contains 4 Transmembrane Domains (TMDs: M1–M4).
  • The M2 transmembrane domain of each subunit lines the central ion pore.
  • Ion selectivity is governed by rings of negatively charged amino acids (glutamate/aspartate) flanking the pore, attracting cations and repelling anions.
  • Channel gating involves a central leucine ring that acts as a physical barrier in the closed state; ACh binding causes a conformational rotation that opens the channel.
• Agonists: Acetylcholine, Nicotine, Methacholine (Mecholyl®), Carbamylcholine (Carbachol®).
• Antagonists: Tubocurarine, Pancuronium, Dextromethorphan, $\alpha$-Bungarotoxin.

Muscarinic Acetylcholine Receptors ($mAChR$)

• Type: G-Protein Coupled Receptors (GPCRs), divided into 5 subtypes ($M_1$–$M_5$).
• Subtype Signaling Pathways:
  • Stimulatory Group ($M_1, M_3, M_5$): Coupled to $G_q$ proteins. Activation stimulates Phospholipase C (PLC), hydrolyzing Phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$) into Inositol 1,4,5-trisphosphate ($\text{IP}_3$) and Diacylglycerol (DAG). $\text{IP}_3$ triggers $Ca^{2+}$ release from the endoplasmic reticulum, while DAG activates Protein Kinase C (PKC).
  • Inhibitory Group ($M_2, M_4$): Coupled to $G_i$ proteins. Activation inhibits Adenylyl Cyclase (AC), decreasing intracellular cyclic AMP (cAMP) levels, closing voltage-gated $Ca^{2+}$ channels, and opening inwardly rectifying $K^+$ channels.
• Agonists: Acetylcholine, Muscarine.
• Antagonists: Atropine.

Specific Signaling Examples

• Postsynaptic $M_3$ Stimulation (Smooth Muscle): Activation of $M_3$ on airway smooth muscle leads to $G_q\rightarrow\text{PLC}\rightarrow\text{IP}_3$-mediated increase in intracellular $Ca^{2+}$. Calcium binds calmodulin, forming a $Ca^{2+}/\text{calmodulin}$ complex that activates myosin light chain kinase (MLCK), enhancing actin-myosin cross-bridge cycling and inducing bronchoconstriction. Long-Acting Muscarinic Antagonists (LAMAs) block $M_3$ receptors, preventing the rise in $Ca^{2+}$ and promoting airway smooth muscle relaxation.
• Presynaptic/Postsynaptic $M_2$ Stimulation (Cardiac Vagal Tone): $M_2$ activation on SA/AV node cardiac cells leads to $G_i$-mediated inhibition of AC (reducing cAMP) and $G_{\beta\gamma}$-mediated activation of $K_{\text{ACh}}$ channels ($K^+$ efflux). This hyperpolarizes the membrane and slows SA node firing, causing bradycardia. Atropine competitively antagonizes $M_2$ receptors, overriding vagal tone and causing tachycardia.

Acetylcholine Synthesis, Storage, and Breakdown

• Chemical Structure: Composed of an acyloxy group, an ethylene bridge, and a quaternary ammonium group containing a positively charged nitrogen atom ($N^+$).
• Biosynthetic Pathway:
  1. Serine undergoes decarboxylation via Serine Decarboxylase to form Ethanolamine.
  2. Ethanolamine is converted to Choline via Choline N-methyltransferase, utilizing S-Adenosylmethionine (SAM) as a methyl donor.
  3. In the cytoplasm of cholinergic axon terminals, Choline and Acetyl-S-Coenzyme A (Acetyl-S-CoA, derived from mitochondrial pyruvate metabolism) are converted into Acetylcholine by the enzyme Choline Acetyltransferase (ChAT).
• Vesicular Storage & Release:
  • ACh is transported into presynaptic vesicles by the Vesicular Acetylcholine Transporter (VAT), driven by a proton antiport gradient.
  • An action potential depolarizes the presynaptic terminal, opening voltage-gated $Ca^{2+}$ channels.
  • Influx of $Ca^{2+}$ causes vesicle fusion with the presynaptic membrane via SNARE proteins (VAMP and SNAP), releasing ACh into the synaptic cleft.
• Metabolism & Termination:
  • ACh is rapidly hydrolyzed in the synaptic cleft by Acetylcholinesterase (AChE) into Choline and Acetate.
  • Free Choline is transported back into the presynaptic terminal via a High-Affinity Sodium-Dependent Choline Transporter (CHT).

Clinical Toxins and Therapeutics Targeting the Cholinergic System

• Hemicholinium: Blocks CHT, inhibiting choline reuptake and depleting ACh stores.
• Vesamicol: Inhibits VAT, preventing ACh entry into synaptic vesicles.
• Botulinum Toxin: Cleaves SNARE proteins (SNAP-25, Synaptobrevin/VAMP), preventing ACh exocytosis and causing flaccid paralysis.
• Peripheral Cholinergic Excess Symptoms:
  • Muscarinic Overstimulation ($mAChR$): Manifests as the SLUDGEM acronym: Salivation, Lacrimation, Urination, Defecation, Gastrointestinal motility/cramping, Emesis, Miosis (plus bradycardia and bronchospasm).
  • Nicotinic Overstimulation ($nAChR$): Sympathetic autonomic ganglion stimulation and NMJ depolarization block result in tachycardia, hypertension, mydriasis, muscle fasciculations, and eventual neuromuscular weakness/paralysis.

The Catecholaminergic Systems: Dopamine, Norepinephrine, and Epinephrine

• Catecholamines are biogenic amines containing a catechol moiety (a benzene ring with two adjacent hydroxyl groups) and an side-chain amine.

Anatomical Distribution and Pathways

• Dopamine (DA):
  • Nigrostriatal Pathway: Cell bodies in the Substantia Nigra pars compacta (SNc) project to the Dorsal Striatum (caudate and putamen); essential for movement initiation and motor fine-tuning.
  • Mesolimbic Pathway: Cell bodies in the Ventral Tegmental Area (VTA) project to the Nucleus Accumbens (nAc, ventral striatum); mediates reinforcement, reward processing, and pleasure.
  • Mesocortical Pathway: VTA neurons project to the Prefrontal Cortex (PFC); governs executive function, working memory, and decision-making.
• Norepinephrine (NE):
  • Cell bodies in the Locus Coeruleus (located within the pontine reticular formation) project to the spinal cord, cerebellum, hypothalamus, thalamus, basal telencephalon, and neocortex.
  • Primary central mediator of vigilance, alertness, arousal, stress responses, and mood regulation.
• Epinephrine (E):
  • Synthesized by scattered neuronal clusters in the medulla oblongata projecting to the hypothalamus and thalamus.
  • Predominantly synthesized and secreted peripheral hormone from the chromaffin cells of the adrenal medulla (located atop the kidneys) under sympathetic control.

Dopamine Receptors and Signaling

• Divided into two GPCR families based on structural homology and second-messenger coupling:

Catecholamine Biosynthetic Pathway

• All catecholamines are derived sequentially from the essential amino acid precursor L-Tyrosine:

$\text{L-Tyrosine} \xrightarrow[\text{TH (Rate-Limiting)}]{O_2, Fe^{2+}, \text{Tetrahydrobiopterin}} \text{L-DOPA} \xrightarrow[\text{AADC / DOPA Decarboxylase}]{\text{Pyridoxal Phosphate (Vit } B_6\text{)}} \text{Dopamine} \xrightarrow[\text{Dopamine } \beta\text{-Hydroxylase}]{O_2, Cu^{2+}, \text{Ascorbic Acid}} \text{Norepinephrine} \xrightarrow[\text{PNMT}]{\text{S-Adenosylmethionine (SAM)}} \text{Epinephrine}$

1. Tyrosine Hydroxylation: L-Tyrosine is oxidized to L-3,4-dihydroxyphenylalanine (L-DOPA) by Tyrosine Hydroxylase (TH) in the neuronal cytoplasm. This step requires molecular oxygen ($O_2$), ferrous iron ($Fe^{2+}$), and tetrahydrobiopterin as cofactors. TH is the rate-limiting enzyme of catecholamine biosynthesis.
2. DOPA Decarboxylation: L-DOPA is converted to Dopamine by Aromatic L-Amino Acid Decarboxylase (AADC) (also known as DOPA Decarboxylase), requiring Vitamin $B_6$ (Pyridoxal Phosphate) as a cofactor. In dopaminergic neurons, the enzymatic pathway terminates at this step.
3. Dopamine $\beta$-Hydroxylation: In noradrenergic neurons, Dopamine is imported into synaptic vesicles via the Vesicular Monoamine Transporter (VMAT), where it is converted to Norepinephrine by Dopamine $\beta$-Hydroxylase. This reaction requires $O_2$, cupric copper ($Cu^{2+}$), and Ascorbic Acid (Vitamin C).
4. N-Methylation: In adrenergic cells containing Phenylethanolamine N-Methyltransferase (PNMT), Norepinephrine exits the vesicle to the cytoplasm and is methylated to Epinephrine utilizing SAM as a methyl group donor.

Dopaminergic and Adrenergic Synaptic Kinetics

• Vesicular Import: Dopamine and Norepinephrine are concentrated into presynaptic vesicles by VMAT2, powered by an inward proton ($H^+$) ATPase gradient.
• Reuptake Mechanisms:
  • Dopamine is cleared from the synaptic cleft via the Dopamine Transporter (DAT), a $Na^+/Cl^-$-dependent symporter.
  • Norepinephrine is cleared via the Norepinephrine Transporter (NET).
• Autoreceptor Regulation:
  • Presynaptic $D_2$ autoreceptors on dopaminergic terminals bind released DA, initiating $G_i$-mediated inhibition of voltage-gated $Ca^{2+}$ channels to decrease further DA exocytosis.
  • Presynaptic $\alpha_2$-adrenergic autoreceptors on noradrenergic terminals bind NE to exert feedback inhibition on NE release.

Catecholamine Catabolism and Metabolites

• Catabolism involves two principal enzymes working sequentially or in parallel: Monoamine Oxidase (MAO) (located on outer mitochondrial membranes) and Catechol-O-Methyltransferase (COMT) (located in cytoplasm and membrane surfaces).

Dopamine Degradation Pathways

• Path A: Dopamine is converted by MAO and Aldehyde Dehydrogenase (AD) into 3,4-Dihydroxyphenylacetic acid (DOPAC), which is then methylated by COMT to form Homovanillic Acid (HVA).
• Path B: Dopamine is methylated by COMT to 3-Methoxytyramine, which is subsequently converted by MAO/AD to Homovanillic Acid (HVA).
• Clinical Relevance: HVA is excreted in the urine and serves as a primary diagnostic biomarker for neuroblastoma (a catecholamine-secreting neuroendocrine tumor).

Norepinephrine Degradation Pathways

• NE is processed by COMT to Normetanephrine or by MAO to 3,4-Dihydroxyphenylglycolaldehyde (DOPGAL).
• Downstream actions by Aldehyde Reductase and Aldehyde Dehydrogenase produce DOPEG, DOMA, MOPGAL, and MOPEG.
• The final end-stage metabolite excreted in the urine is Vanillylmandelic Acid (VMA).

Pharmacology and Pathology of Catecholamines

• Parkinson's Disease: Progressive degeneration of dopaminergic neurons in the SNc leading to striatal DA deficiency, causing akinesia, rigidity, and resting tremor. Treated with L-DOPA (precursor that crosses the blood-brain barrier), Selegiline (MAO-B inhibitor), or Ropinirole ($D_2/D_3$ agonist).
• Schizophrenia: Associated with subcortical dopaminergic hyperfunction. Managed with $D_2$ receptor antagonists such as Haloperidol.
• Cocaine: Blocks DAT and NET reuptake transporters, preventing clearance of DA and NE from the synaptic cleft, leading to persistent postsynaptic activation.
• Reserpine: Irreversibly blocks VMAT2, preventing monoamine import into synaptic vesicles. Unstored cytosolic catecholamines are degraded by mitochondrial MAO, causing long-term depletion of monoamine transmitter stores.

The Histaminergic System

Anatomy and Physiological Roles

• Cell bodies of histaminergic neurons are located exclusively in the Tuberomamillary Nucleus (TMN) of the posterior hypothalamus.
• TMN axons project diffusely throughout the central nervous system, including the cerebral cortex, thalamus, brainstem, and spinal cord.
• Central Functions: Maintenance of wakefulness, behavioral arousal, focused attention, and suppression of REM sleep.
• Peripheral Functions: Stimulation of gastric acid secretion by parietal cells, immunomodulation, bronchial smooth muscle constriction, and microvascular vasodilation.

Histamine Receptors

• Three histamine receptor subtypes are expressed in the CNS ($H_1, H_2, H_3$):
  • $H_1$ Receptor: Coupled to $G_q\rightarrow\text{PLC}\rightarrow\text{IP}_3/\text{DAG}\rightarrow\uparrow Ca^{2+}$; excitatory receptor promoting cortical wakefulness.
  • $H_2$ Receptor: Coupled to $G_s\rightarrow\text{Adenylyl Cyclase}\rightarrow\uparrow\text{cAMP}$; excitatory.
  • $H_3$ Receptor: Coupled to $G_i\rightarrow\text{Adenylyl Cyclase}\rightarrow\downarrow\text{cAMP}$; acts as an inhibitory autoreceptor on TMN terminals to suppress histamine release.

Synthesis, Storage, Breakdown, and Pharmacology

• Synthesis: L-Histidine enters the terminal via an L-amino acid transporter and is decarboxylated by Histidine Decarboxylase to yield Histamine.
• Storage & Metabolism: Loaded into vesicles via VMAT. Cleared from the extracellular space and converted by Histamine Methyltransferase to Tele-methylhistamine, or degraded by Diamine Oxidase.
• Pharmacological Interference:
  • Diphenhydramine: $H_1$ receptor antagonist that crosses the blood-brain barrier, blocking central excitatory $H_1$ signals and producing sedation.
  • Propofol: Intravenous anesthetic that directly inhibits the electrical firing of TMN histaminergic neurons, causing loss of consciousness.

The Indolaminergic and Melatonergic Systems: Serotonin and Melatonin

Serotonin ($5\text{-HT}$)

Anatomical Distribution

• Central Nervous System: Cell bodies clustered in the Raphe Nuclei (located along the midline of the brainstem/pons) project to the hypothalamus, limbic system, neocortex, cerebellum, and spinal cord. Modulates mood, affect, anxiety, cognitive control, impulse control, and nociceptive pain processing in the dorsal horn of the spinal cord.
• Gastrointestinal Tract: Over 90% of body serotonin is synthesized by Enterochromaffin (EC) cells in the gut mucosa, regulating peristalsis, fluid secretion, and nausea sensory afferents.
• Platelets: Uptake serotonin from plasma for release during platelet aggregation to mediate localized vascular responses.

Serotonin Receptors

• Comprises 15 identified receptor subtypes grouped into 7 distinct families ($5\text{-HT}_1$ through $5\text{-HT}_7$).
• Ionotropic Receptor:
  • $5\text{-HT}_3$: Non-selective ligand-gated cation channel ($Na^+, K^+, Ca^{2+}$ permeable); mediates rapid depolarization in the chemoreceptor trigger zone (CTZ) and gut afferents.
• Metabotropic Receptors:
  • $5\text{-HT}<em>1\text{ Family } (5\text{-HT}</em>{1A}, 5\text{-HT}<em>{1B}, 5\text{-HT}</em>{1D})\text{ and } 5\text{-HT}<em>5$: Coupled to $G</em>{i/o}$, inhibiting Adenylyl Cyclase ($\downarrow\text{cAMP}$). $5\text{-HT}_{1B/1D}$ function as presynaptic autoreceptors.
  • $5\text{-HT}<em>2\text{ Family } (5\text{-HT}</em>{2A}, 5\text{-HT}<em>{2B}, 5\text{-HT}</em>{2C}$): Coupled to $G_{q/11}$, stimulating Phospholipase C ($\uparrow\text{IP}_3/\text{DAG}/Ca^{2+}$).
  • $5\text{-HT}_4, 5\text{-HT}_6, 5\text{-HT}_7\text{ Families}$: Coupled to $G_s$, activating Adenylyl Cyclase ($\uparrow\text{cAMP}$).

Biosynthesis and Catabolism

• Serotonin features an indole ring core (benzene ring fused to a pyrrole ring).

$\text{L-Tryptophan} \xrightarrow[\text{Tryptophan Hydroxylase (TPH - Rate-Limiting)}]{O_2, \text{Tetrahydrobiopterin}} \text{5-Hydroxytryptophan (5-HTP)} \xrightarrow[\text{AADC}]{\text{Pyridoxal Phosphate (Vit } B_6\text{)}} \text{5-Hydroxytryptamine (5-HT)}$

1. Hydroxylation: L-Tryptophan is converted to 5-HTP by Tryptophan Hydroxylase (TPH); this step is rate-limiting.
2. Decarboxylation: 5-HTP is converted to 5-HT (Serotonin) by AADC.

• Metabolism: Serotonin is imported into presynaptic terminals via the Serotonin Transporter (SERT) and degraded by MAO and Aldehyde Dehydrogenase to 5-Hydroxyindoleacetic Acid (5-HIAA), which is excreted in the urine.

Serotonergic Pharmacology

• Selective Serotonin Reuptake Inhibitors (SSRIs): Block SERT, elevating synaptic 5-HT levels (e.g., Escitalopram, Fluoxetine, Paroxetine, Citalopram).
• Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs) & Tricyclic Antidepressants (TCAs): Dual inhibition of SERT and NET (e.g., Duloxetine, Venlafaxine, Amitriptyline, Desipramine, Imipramine).
• $5\text{-HT}_3$ Antagonists: Antiemetics that block ionotropic $5\text{-HT}_3$ receptors in the gut and solitary tract nucleus to treat chemotherapy-induced nausea and vomiting (e.g., Ondansetron).

Melatonin

• Synthesized in the Pineal Gland from Serotonin via a two-step enzymatic process:

$\text{Serotonin} \xrightarrow[\text{Serotonin N-Acetyltransferase (NAT)}]{} \text{N-Acetylserotonin} \xrightarrow[\text{Hydroxyindole O-Methyltransferase (HIOMT)}]{} \text{Melatonin}$

• Physiology: Secretion is controlled by light exposure via the Suprachiasmatic Nucleus (SCN) of the hypothalamus; light suppresses NAT activity, whereas darkness promotes synthesis. Melatonin diffuses into the circulation to coordinate circadian sleep-wake architecture.
• Receptors: $MT_1$ and $MT_2$ receptors (both $G_{i/o}$-coupled GPCRs) expressed in the SCN, retina, and anterior pituitary.
• Pharmacology:
  • Ramelteon: Selective $MT_1/MT_2$ agonist prescribed for sleep-onset insomnia.
  • Agomelatine: $MT_1/MT_2$ receptor agonist and $5\text{-HT}_{2C}$ receptor antagonist used to treat major depressive disorder and seasonal affective disorder (SAD).

Amino Acid Neurotransmitter Systems: GABA, Glycine, Glutamate, and Aspartate

Inhibitory Amino Acid Neurotransmitters

$\gamma$-Aminobutyric Acid (GABA)

• Primary inhibitory neurotransmitter in the central nervous system.
• Biosynthesis and Recycled Glutamine Cycle:
  • Synthesized directly from L-Glutamate by the enzyme Glutamic Acid Decarboxylase (GAD), utilizing Pyridoxal Phosphate (Vitamin $B_6$) as an essential cofactor.
  • Released GABA is removed from the cleft by GABA Transporters (GATs) into presynaptic terminals or surrounding astrocytes.
  • In astrocytes, GABA is catabolized by GABA Transaminase (GABA-T) into Succinic Semialdehyde, which is converted by Succinic Semialdehyde Dehydrogenase (SSADH) to Succinic Acid (entering the Krebs cycle).
  • Alternatively, $\alpha$-ketoglutarate from the Krebs cycle is transaminated by GABA-T to synthesize Glutamate, which is converted to Glutamine by Glutamine Synthetase in glial cells, transported back to neurons, and hydrolyzed to Glutamate by Glutaminase.
• Pharmacological Target: Vigabatrin irreversibly inhibits GABA-T, blocking GABA degradation and increasing brain GABA concentrations.

GABA Receptors

• $GABA_A$ Receptor:
  • Heteropentameric ligand-gated $Cl^-$ ion channel.
  • $GABA$ binding causes rapid opening of the central channel pore, allowing $Cl^-$ influx, postsynaptic hyperpolarization, and membrane shunt inhibition.
  • Allosteric Modulatory Sites:
  • GABA Binding Site: Orthosteric activation site (blocked by competitive antagonists).
  • Benzodiazepine Site (+): Positive allosteric modulator; increases channel opening frequency in the presence of GABA.
  • Flumazenil (-): Neutral competitive antagonist at the benzodiazepine binding site; reverses benzodiazepine overdose.
  • Barbiturate Site (+): Positive allosteric modulator; increases channel opening duration; at high concentrations can directly gate the channel.
  • Neurosteroid Site (+): Enhances channel activity.
  • Picrotoxin Site (-): Non-competitive channel blocker that physically plugs the pore.
  • Other Inhibitors (-): Penicillin, Furosemide.

• $GABA_B$ Receptor:
  • Metabotropic $G_{i/o}$-coupled GPCR.
  • $G_{\alpha}$ subunit inhibits Adenylyl Cyclase; freed $G_{\beta\gamma}$ subunits directly open inwardly rectifying $K^+$ channels ($K^+$ efflux) and close voltage-gated $Ca^{2+}$ channels, producing slow, prolonged inhibitory postsynaptic potentials.

Glycine

• Primary inhibitory neurotransmitter in the spinal cord, brainstem, and lower brain regions.
• Biosynthesis: Synthesized from Glucose $\rightarrow$ Serine, which is converted to Glycine by Serine Hydroxymethyltransferase.
• Receptor: Ionotropic $Cl^-$ channel receptor causing membrane hyperpolarization.

Excitatory Amino Acid Neurotransmitters

Glutamate and Aspartate

• Glutamate is the principal excitatory neurotransmitter throughout the entire CNS.

Ionotropic Glutamate Receptors

• Tetrameric integral membrane protein complexes gating cation-permeable channels:
  • AMPA Receptors ($\alpha$-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid): Heterotetramers permeable to $Na^+$ and $K^+$. Mediates rapid, transient synaptic depolarization. Modulated allosterically by alcohols, volatile anesthetics, and barbiturates.
  • Kainate Receptors: Similar to AMPA receptors; gate $Na^+$ and $K^+$ flux.
  • NMDA Receptors (N-methyl-D-aspartate):
  • High permeability to $Ca^{2+}$, as well as $Na^+$ and $K^+$.
  • Dual Gating Mechanism: Requires simultaneous binding of Glutamate and the obligate co-agonist Glycine.
  • Voltage-Dependent $Mg^{2+}$ Block: At resting membrane potentials (e.g., $-70\,\text{mV}$), a hydrated $Mg^{2+}$ ion lodges within the outer channel pore, blocking ion flow even when glutamate and glycine are bound.
  • Postsynaptic membrane depolarization (mediated by adjacent AMPA receptor activation) electrostatically expels $Mg^{2+}$ from the pore, allowing $Ca^{2+}$ and $Na^+$ influx.
  • Modulatory Sites: Inhibited by extracellular Zinc ($Zn^{2+}$) and uncompetitive channel blockers such as Phencyclidine (PCP).

Metabotropic Glutamate Receptors ($mGluR$)

• GPCRs divided into 8 subtypes across 3 groups:
  • Group I ($mGluR_1, mGluR_5$): Coupled to $G_q\rightarrow\text{PLC}\rightarrow\uparrow Ca^{2+}$; increases postsynaptic excitability.
  • Group II ($mGluR_2, mGluR_3$) and Group III ($mGluR_4, mGluR_6, mGluR_7, mGluR_8$): Coupled to $G_i/G_o\rightarrow\downarrow\text{cAMP}$; act primarily as presynaptic autoreceptors to inhibit glutamate exocytosis.

Synaptic Interaction Model of Glutamate Receptors

1. Presynaptic exocytosis releases Glutamate into the cleft.
2. Glutamate binds AMPA receptors, causing $Na^+$ influx and local postsynaptic depolarization.
3. Membrane depolarization removes the voltage-dependent $Mg^{2+}$ block from NMDA receptors.
4. Glutamate and co-agonist Glycine activate unblocked NMDA receptors, causing $Ca^{2+}$ influx.
5. Intracellular $Ca^{2+}$ activates downstream protein kinases (e.g., CaMKII, PKC), promoting insertion of additional AMPA receptors into the postsynaptic density and triggering the release of retrograde messengers (e.g., Nitric Oxide) that enhance presynaptic neurotransmitter release probability.

Comprehensive Reference of Neurotransmitter Pathways and Features